Zinc base alloy casting



United States Patent ZINC BASE ALLOY CASTING Leslie Larrieu, San Marino, Ca]if., assignor to Morris P. Kiri: 8: Son, Inc., Los Angeles, Ca.lif., a corporation of California No Drawing. Filed May 19, 1958, Ser. No. 735,965 10 Claims. (Cl. 75-178) This invention relates to zinc base alloys containing minor arnounts of aluminum and copper, and particularly to the production of sand castings from such alloys, especially sand-cast forming dies.

Such dies are used as forming tools in drop hammers, hydro-presses, and other press-forming equipment such as stamp, forging, trimming, blanking and piercing presses employed for shape-forming of wrought sheet alloys, especially in the aircraft, automobile and related industries.

The quality of sand castings of zinc base alloys has been relatively poor because of the coarse grain structure and lower density of the castings produced by sand casting methods. In the production of sand castings the rate of solidification is relatively slow and the coarse grain structure produced results in decreased mechanical properties of the castings. This is especially true in the production of large castings weighing as much as several tons which necessitate the use of special techniques, including delayed cooling periods, to promote soundness throughout the entire structure of the casting. it is known that dies produced by sand-casting methods sometimes fail prematurely by developing cracks because the stresses encountered during their use in forming operations exceed the reduced mechanical properties of the coarse grain structure or" the sandcast die. Sand-cast zinc base alloy dies are known to have relatively poor ductility and reduced tensile strength characteristics.

Accordingly, it is an object of my invention to provide a zinc base alloy, casting and method which are free of the above described disadvantages and defects.

Another object is to provide a zinc base alloy sandcasting such as a sand-cast forming die, for example, having increased mechanical properties.

A further object is to provide a sand-cast die of zinc base alloy having refined grain structure and improved impact strength, ductility and tensile strength characteristics.

Additional objects and advantages of my invention will become apparent from the following description which is given primarily for purposes or illustration and not limitation.

I have discovered that when the amount of magnesium in a zinc base, aluminum and copper containing alloy is kept very low, such as in the range of a'trace to about 0.03 percent by weight based on the mixture, containing aluminum in the range of about 3.5 to about 4.8 percent, copper in the range of about 2.0 to about 4.0 percent, and the remainder pure zinc, a remarkable improvement in the grain size and mechanical properties of sand castings produced therefrom. especially sand-cast forming dies, is achieved without significantly affecting the fluidity or castability of the alloy for casting in sand molds and without noticeably affecting the intergranular corrosion of the sand castings produced from the alloy. Even better results are obtained, particularly when the alloy is used to produce sand-cast forming dies, when the magnesium content of the alloy is kept in the range of a trace to about 0.01 percent, aluminum in the range of about 4.0

to about 4.4 percent, copper in the range of about 3.0 to about 3.3 percent, and the remainder pure zinc. A preferred composition contains about 4.25 percent aluminum, about 3.20 percent copper, a trace to about 0.005 percent magnesium, and the remainder pure zinc.

The following tables illustrate the important mechanical properties of preferred alloys of my invention in the sandcast state compared with those of A.S.T.M. Alloy XXI in the sand-cast state. The typical mechanical results tabu- Properties in sand-cast state A.S.T.NI. Alloy XXI Improved Alloy Tensile strength 34,000 psi. 37,000 p.s.i.

Elongation Brinell hardness 110. Impact strength (Charpy /4" it Mr bar) 6 ft. lbs 25 ft lbs. Grain size Medium fine Ultra fine.

It wiil be observed that the impact strength of the improved sand-cast alloy of my invention is more than four times that of A.S.T.M. Alloy XXI and that the tensile strength, elongation and hardness of the former cast alloy are improved over the latter. The grain size of the metal in sand castings of the improved alloy is much finer than in the A.S.T.M. Alloy XXI. The extension of the improvement in mechanical properties and refinement in grain size to large masses of sand cast alloy has been verified by observing improved service life or" sand-cast forming dies produced from the improved alloy of my invention and by observation of the much finer grain size in pieces broken from such sand-cast forming dies.

Thus, the more important objects of the invention'are accomplished by producing a sand-cast zinc base alloy containing a preferred composition of several percent of aluminum and copper and substantially no magnesium, tin, lead, cadmium or other impurities. Such an alloy possesses greatly refined grain structure, substantially .increased impact strength and other mechanical properties without noticeable increase in undesirable properties such as intergranular corrosion. Small amounts of magnesium are detrimental in an alloy used to produce sand castings because the magnesium appears to cause a coarse grain structure and reduced mechanical properties, as shown by the comparison of properties given above in tabulated form.

The zinc base alloy of my invention should not be confused with the zinc base alloys of the prior art developed for pressure die casting purposes. These alloys contained very small amounts of lead, tin, cadmium and bismuth which tended to produce intergranular corrosion in the fine grained castings produced by pressure die casting methods. It was found that the addition of a small amount of magnesium to these die casting alloys substantially reduced the tendency toward intergranular corrosion or intercrystalline oxidatiod'in the presence of warmth and moisture together with attend.-

upon the same alloy when sand-cast.

which were developed primarily for use in the production of sand castings, especially sand-cast forming dies, as dis- ,tinguished from die castings which are cooled rapidly in practice and inherently have finer grain structures than sand castings, which, asexplained hereinabove, inher ently have relatively coarse grain structures because of their relatively slow rate of cooling.

Furthermore, it is known that the same amount of lead that causes intergranular' corrosion or intercrystah line oxidation in a zinc base, aluminum and copper contaiiiing casting produced by pressure die casting methods will on the contrary have very little similar effect This difference in properties'between the die casting and the sand casting produced from the same alloy is attributable to grain size. Intergranular corrosion in zinc base alloys is be- 1 lievedto take place within the alloy upon aging by the migration of the responsible elements such as lead, tin and cadmium to the grain boundaries where they are oxidized and through resultant volume change disrupt the ing method, a powerful expansive force is exerted by the -oxidation of the softmetal impurities such as lead, which have accumulated at the relatively larger total grain boundary areasproduced by the fine grain structure, as compared-to the smaller total grain boundary areas for a-given mass of the coarser grained sand-cast structure. "For example, it has been demonstrated in'the case of lead contaminated aircraft forming dies produced by a homogeneous mixture.

exercised to avoid including impurities in the alloy. The

inclusion of iron, for example, from the melting con-' tainers, can be minimized by Iollowing a carefully planned procedure designed to keep the temperatures of the molten metals at a minimum. I prefer -to make a melt of predetermined amounts of aluminum and copper to form a hardener alloy. If any magnesium is to be added to the casting alloy, the predetermined amount of magnesium to be added is included with the aluminum and copper in thehardener 'alloy' melt; .1 The'frnelt of hardener alloy components "is prepared at the lowest temperature at which it is practical to prepare the same. The hardener alloy contains. about 56 to about 58'percen aluminum 1 and about 42 to about '44 percent copper. If magnesium is present inthe hardener a'lloy,'iit amounts to about 0.05 to 0.40 percent by'weight of the mixture. The hardener alloy mixture is heated to about 1300 F. until the aluminum commences to melt. The temperature then isdropped to about 12 00 F. until the aluminum and copper (and magnesium, any) have melted to form The hardener alloy then is cast by pouring the melt into molds that produce small sized alloy, In the case of ultra fine grained cast structures, suchas those obtained by the quick chill of the die castingots with large surface areas to form a casting of fine grain structure,or 'is added directly. to a predetermined amount of molten zinc if the castingoi the final structure is to take place at the timeand place of forming the hardener alloy.

Otherwise the hardener alloy is transported to the location where the casting is to take place, The predetermined amount of purezinc is melted by heating it to a temperature of about 900 F. until the zinc is completely liquid. Then the temperature is slightly increased to about 950 F. and maintained at that value during the I addition of a predetermined amount of the fine grained sand-casting'methods that the coarse grained structures thereof do'not have the high susceptibility to intergranular corrosion or oxidation exhibited by fine grain diecast structures produced from the same alloys. In par- .;ticular, it has been found that in zinc base alloys containing several percent aluminum and copper, magnesium contents ranging from about 0.005 to about 0.14 percent, do not cause a noticeable reduction in intergranular corrosion in sand-cast structures, whereas it is known that they cause a substantial reduction in intergranular corrosion in pressure die-cast structures. The

. larger grainsin the sand-cast-structures are characterized by greater migration distances within each grain and smaller total grain boundary are a per unit of mass than the smaller grains of the pressure die-cast structures with the result that the larger grains of the sand-cast structures exhibit much less intergr'anular or intercrystalline'oxidation and much less destructive expansion than the die cast structures.

Thus the addition of magnesium to such alloys used to produce sand-cast structures does not produce a notice- -able beneficial effect. Furthermore, it has been found that such addition of magnesium to such zinc base a1- loys actually is detrimental; Although the-relatively enlarged grain'siz'e in the sand-cast structures is primarily due tofl the slow, cooling rate inherent in sand-casting pause a coarsening of the grain size in the cast.struc- "iture together with acorresponding reduction in its mechanical properties such/as elongation, tensile strength and impact strength.

' 'In preparing'the alloy 'ror use in producing sand cast ings in accordance with my invention, care shrould be intergranular corrosion-in such structures,{but actually hardener alloy.. The temperature of the mixture then is adjusted 'to about 825 F. and maintained at that value until all of the metal is a' homogeneous mixture. The resulting melt then is fed to a suitable mold made of sand to produce a sandcasting of the desired form, such as a forming die, for example, or the melt is cast into suitable ingots for subsequent remelting and casting in sand molds. V

Forming dies producedby the method of my invention have been found to stand up better under actual service conditions than sand-cast dies containing about 0.03 percent magnesium. They have not failed prematurely by developing cracks because they consisted of finer grain structure'rnetal. They also showed superior ductility, impact strength and hardness,

I claim as myinvention: r

1. A zinc base sand cast alloy comprising about 3.5% to 4.8% aluminum by w eigh about 2.0 to about 4.0% copper by weight, from a trace to about 0.03% magnesium by weight, and the remainder zinc.

2. A zinc base sand cast alloy comprising aluminum in the range ofabout 4.0% to about 4.4% by weight, copper in the range of about 3.0% to about 3.3% by weight, magnesium in the range ofa trace to about 0.01% by weight,. and the remainder zinc. 3. A zinc base sand cast alloy comprising about 4.25% aluminum by weight, about 3.20% copper by weight, about 0.005% magnesium by weight, and the'remainder ZlIlC.

4. .A formin tool composed of a sand cast alloy conabout 2.0% to about "4.0% ,copper by weight, from a trace to about.0.03% magnesium by weight, and the re-- mainder zinc.

. 5. A forming tool composed of a sand cast'alloy containing aluminum in the range. of about 4.0% to. about 4.4% by. weight, copper in the range ofabout 3.0% to about 3.3 by weight, magnesium'in the range of a trace to about 0.01% by weight, and theremainder zinc.

. 6. A. forming tool composed of a -sand castjallov' comprising about 4.25% aluminum by weight, about 3.20% copper by weight, about 0.005% magnesium by weight, and the remainder zinc.

7. A forming die composed of a sand cast alloy containing aluminum in the range of about 4.0% to about 4.4% by weight, copper in the range of about 3.0% to about 3.3% by weight, magnesium in the range of a trace to about 0.01% by Weight, and the remainder zinc.

8. A zinc base sand cast alloy comprising about 3.5% to about 4.8% aluminum by weight, about 2.0% to about 4.0% copper by weight, from a trace to about 0.03% magnesium by Weight, and the remainder zinc characterized by a fine grain structure and a high impact strength.

9. A zinc base sand cast alloy comprising aluminum in the range of about 4.0% to about 4.4% by weight, 15

copper in the range of about 3.0% to about 3.3% by weight, magnesium in the range of a trace to about 0.01%

by weight, and the remainder zinc, characterized by a fine grain structure and a high impact strength.

10. A forming tool composed of a sand cast alloy comprising about 4.25% aluminum by weight, about 3.20% copper by weight, about 0.005% magnesium by weight, and the remainder zinc, characterized by a high impact strength of the order of 25 ft. lbs. and a Charpy x A; bar and extremely fine grain structure.

References Cited in the file of this patent UNITED STATES PATENTS 1,540,006 Hodson June 2, 1925 2,589,398 Kornblum Mar. 18, 1952 FOREIGN PATENTS 378,645 Great Britain Aug. 18, 1932 

1. A ZINC BASE SAND CAST ALLOY COMPRISING ABOUT 3.5% TO 4.8% ALUMINUM BY WEIGHT, ABOUT 2.0 TO ABOUT 4.0% COPPER BY WEIGHT, FROM A TRACE TO ABOUT 0.03% MAGNESIUM BY WEIGHT, AND THE REMAINDER ZINC. 